Method for producing hollow investment castings

ABSTRACT

A method for casting metering holes connecting the inner and outer surfaces of hollow investment cast objects, such as hollow air cooled gas turbine airfoils. The method incorporates mini cores having metering hole pins which extend further into the inner cavity than the surface of the cavity, so that the ends of the holes can be exposed by a subsequent machining operation. The method also includes forming a wax pattern which avoids contact between the wax and the metering hole pins so that the pins are not broken by the wax during the heatup portion of the wax removal step after formation of the ceramic casting mold.

TECHNICAL FIELD

The present invention relates to manufacture of cast articles havinginternal passages therein, and relates particularly but not exclusivelyto a method of casting turbine airfoils with internal cooling passagestherein.

BACKGROUND ART

Investment casting is a well-known technique for producing articleshaving, among other features, internal cavities. The cavities may benecessary for weight reduction, containment capacity or flow-throughcapability. The investment casting process has been found to be veryuseful for fabrication of complex metal castings, especially thosehaving hollow internal cavities. By properly supporting patterns made ofan easily removable substance, such as wax, very complex internalconfigurations can be produced.

Gas turbine engines utilize hollow components, primarily for weightreduction and for cooling capability. Cooling is achieved by flowingbleed air through some of the components, particularly airfoils such asblades and vanes in the turbine section, where the highest operatingtemperatures are encountered, and where the efficiency of the engine ismost limited by the capability of the materials to withstand the effectsof high temperatures. By appropriate cooling, the operating temperaturescan be raised to levels which would otherwise destroy, or severelyshorten the lifetime of, uncooled components. A typical air cooled vaneis shown in FIG. 1.

In addition to flowthrough cooling, air is frequently bled from theinternal cavity through the airfoil walls so that it flows over theouter surface of the airfoil to provide film cooling. Common methods offorming the air outlets through the airfoil walls include electron beamand laser drilling, and electrical discharge machining (EDM). Whilethese techniques have been successfully employed for many years, thecooling passages are essentially restricted to a line-of-sightconfiguration. They also require extra manufacturing steps, involvingtime-consuming and labor intensive processes, and are thus veryexpensive.

Techniques have been developed whereby ceramic mini cores are embeddedin the wax patterns so that, when the wax patterns are removed afterformation of the ceramic mold around the wax pattern, the mini coresremain as part of the mold and define the pathways through the airfoilcomponent by which the cooling air flows from the inner cavity of theairfoil to its outer surfaces.

To achieve the proper cooling of the airfoil without diverting excessiveincoming air, which would adversely affect the efficiency of engineoperation, very close tolerance metering holes are required to controlthe amount of air flowing through the cooling passages.

Initial attempts at casting the metering holes, using extensions of themini core which form the metering holes, hereinafter called meteringhole pins, resulted in excessive breakage of the metering hole pins,which seemed to occur during the wax removal portion of the moldfabrication process. These difficulties encountered with casting in ofthe metering holes initially dictated that the metering holes be formedafter the casting process, generally by an EDM technique. Again theextra manufacturing steps required are time consuming and expensive.

Thus it is necessary to have a method of casting in the metering holesso that a simple machining operation opens the holes to air flow. It isfurther necessary to have a method which permits formation of theinvestment casting mold without damaging the metering hole pins.

DISCLOSURE OF INVENTION

The present invention solves these problems by eliminating the contactbetween the metering hole pins and the wax which forms the pattern forcasting the airfoil. The invention process incorporates a wax pattern inwhich the receptacle for receiving the mini cores has an enlargedportion into which the metering hole pins are placed without contactingthe wax pattern. The mini cores are then sealed in position in thereceptacles so that the ceramic slurry, from which the mold is made,does not flow into the metering hole pin receptacle and surround themetering hole pins during the mold making operation. This assures thatthe mold material will not contact the metering hole pins, a requirementwhich would otherwise have to be satisfied by the wax pattern.

The enlarged pin cavity is shaped such that, after the wax pattern isremoved, commonly by heating to melt or burn it out, and the investmentcasting is made, metal will completely surround the pins in the enlargedpin receptacle portion and form protrusions on the inner surface of thehollow casting. The pins which form the metering holes are completelycovered by metal during the casting process. The length of the pins issufficient that the metering holes formed by the pins during the castingprocess extend toward the center of the hollow airfoil beyond the finishdimension of the airfoil cavity. Removal of all or a portion of theprotrusions by any of several common techniques exposes and opens theblind ends of the metering holes, thus opening the pathway for flow ofcooling air from the inside to the outside surfaces of the airfoil.

These, and other features and advantages of the invention, will beapparent from the description of the Best Mode, read in conjunction withthe drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a typical hollow air cooled gas turbineengine turbine vane.

FIG. 2 is a partial cross section of the mold for forming the waxpattern.

FIG. 3 is a partial cross section showing the positioning of the minicores in the wax pattern.

FIG. 4 is a partially sectioned perspective view of a wax pattern withmini cores in position after the ceramic mold has been formed.

BEST MODE FOR CARRYING OUT THE INVENTION

The essential feature of this invention is the technique developed forformation of the metering holes which connect the inner and outerairfoil surfaces and control the flow rate of cooling air through thecooling passages. This involves protection of the metering hole pins onthe mini core during the mold formation process, and the opening of themetering holes to airflow after the airfoil has been cast. By protectingthe pins, the metering holes can be formed during the airfoil castingprocess, and opened up by a simple machining operation, rather thanrequiring a separate set of complex operations to machine in themetering holes after the airfoil component has been cast.

Reference is now made to FIG. 2, which shows how the wax pattern 10 isformed with receptacles 12 for mini cores, which ultimately define theconfiguration of the cooling passages. The inner ceramic core 14 hasdepressions 16. The wax pattern mold 18 has protrusions 20 which definethe receptacles 12 for the mini cores, and includes extensions 22 of theprotrusions 20 which form the enlarged receptacle portions which acceptthe metering hole pins. The protrusion extensions 22 also serve aslocating pins to assure that the inner core 14 and the wax pattern mold18 are held in proper relation to each other during formation of the waxpattern 10. The wax pattern 10 is formed by pouring molten wax into thespace between the core 14 and the mold 18.

Referring to FIG. 3, the mini core receptacles 12 are made to provide afairly tight fit when the mini cores 24 are inserted. The enlargedportions 22 of the receptacles 12, which accept the metering hole pins26 on the mini cores 24, must be large enough to avoid contact betweenthe pins 26 and the wax pattern 10. This provides a relief zone aroundthe pins 26 so that, after the ceramic mold has been formed around thewax pattern 10, forces generated by expansion of the wax during theheatup portion of the wax removal process are not transmitted to thepins. The mini cores 24 are then held in place by the wax cover plates28, which are "cemented" in place using molten pattern wax. This coverplate 28 defines a portion of the airfoil outer surface, holds the minicore 24 in position, establishes the thickness of the airfoil wall overthe cooling passage, and seals around the mini core 24 to prevent anyflow of ceramic mold material into the enlarged portion 22 of thereceptacle 12. The extended portions 30 of the mini cores 24 havegeometric features 32 which assure that the mini cores 24 are heldfirmly in place by the ceramic material when the ceramic mold is formed.

Referring now to FIG. 4, the wax pattern 10 has been formed around thenecessary internal cores 14, and the mini cores 24 have been installedand fastened in place. The figure shows, for illustrative purposes, aportion of mini core receptacle which has not been filled, and minicores which are only partially covered with the ceramic mold material toshow how the ceramic locks around the extended portion of the minicores. The assembly is dipped repeatedly into a slurry of ceramic moldmaterial until a ceramic mold 34 of sufficient thickness has been builtup. Appropriate additions of a stucco-like material are incorporatedinto the ceramic mold material to provide additional thickness necessaryfor mold wall strength and resistance to deformation at the elevatedtemperatures incurred during the casting process. The wax pattern 10 isthen removed, generally by heating to melt or burn out the wax.

Molten metal is then poured into the mold and flows into the cavity leftby the removal of the wax pattern. After the metal has solidified, theinternal cores and the mini cores are removed by a chemical leachingprocess which dissolves the core material, leaving the hollow metalcasting with the cooling passages in place.

Removal of the excess material on the protrusions which extend beyondthe ends of the metering flow pins by an EDM process exposes the ends ofthe flow holes and opens them up to air flow.

The process of the present invention may be better understood throughreference to the following illustrative example.

EXAMPLE I

A wax pattern was prepared for a test piece simulating a wall of ahollow airfoil. The pattern incorporated receptacles for cooling passagemini cores, including enlarged portions into which thin pins extended.The wax pattern was prepared on a substrate which had indentations whichformed protrusions surrounding the enlarged receptacle portions.

A ceramic mold was then formed around the wax pattern, and the mold washeated to remove the wax. A casting was then made using a nickel basesuperalloy, PWA 1484, having a nominal composition of 5.0 Cr, 10 Co, 1.9Mo, 5.9 W, 3.0 Re, 8.7 Ta, 5.65 Al, 0.10 Hf, balance Ni, where thestandard chemical symbols represent the weight percent of each elementin the alloy.

After removal of the ceramic mold and cores, it was determined that allof the small pins on the mini cores had survived the mold makingprocess, and formed holes in the metal casting. The excess material onthe protrusions was then removed by EDM, and the holes were opened topermit flow.

A similar attempt to cast a sample with the wax pattern in intimatecontact with the pins resulted in breakage of approximately 60% of thepins, with consequent failure to produce holes in those locations duringthe casting process.

EXAMPLE II

A gas turbine engine turbine vane was fabricated using proceduressimilar to those employed in Example I. In this case, the appropriatecores for the vane cavity and cooling passages were incorporated intothe wax pattern. The wax pattern was then formed with enlargedreceptacles for the metering hole pins on the mini cores, andprotrusions into the vane cavity coincident with the pins on each minicore.

A vane was then cast using PWA 1484 as in the previous example. Afterremoval of the cores, the excess portions of the protrusions in thecavity of the vane were removed by EDM. It was determined that all ofthe metering hole pins had remained intact during the wax removal andcasting processes, and the metering holes had been successfully formedduring the casting operation.

Although this process has been described in terms of its application togas turbine hardware, one of average skill in the art will understandthat the principles are applicable to many other situations in whichsmall features on the core can be broken off during the formation ofinvestment casting molds, and that various changes, omissions andadditions in form and detail thereof may be made without departing fromthe spirit and scope of the claimed invention.

We claim:
 1. A method for fabrication of a hollow air cooled investment cast gas turbine airfoil component comprising:a. making a wax pattern having receptacles for mini cores including enlarged receptacle portions to accept pins on the mini cores which form passages connecting the inner surface and the outer surface of the airfoil component, the pin receptacle extending beyond the internal finish dimension of the hollow airfoil component; b. making the mini cores which have pins longer than necessary to form the required length of the connecting passages; c. fastening the mini cores in position in the wax pattern with no openings between the pin receptacles and the outer surface of the wax pattern; d. forming a ceramic mold surrounding the wax pattern; e. removing the wax pattern from the ceramic mold; f. casting the hollow airfoil component in the ceramic mold, thus forming a casting having protrusions with excess material in the pin cavity portions; g. removing the hollow airfoil component from the ceramic mold; h. removing the mini cores from the hollow airfoil component; and i. removing the excess material from the protrusions, thus exposing the ends of the connecting holes.
 2. A method for fabrication of a hollow investment casting having passages connecting the inner surface and the outer surface of the casting comprising:a. making a wax pattern having receptacles for mini cores including enlarged receptacle portions to accept pins on the mini cores which form the connecting passages, the pin receptacle portions extending beyond the internal finish dimension of the hollow casting; b. making the mini cores, which have pins longer than necessary to form the required length of the connecting passages; c. fastening the mini cores in position in the wax pattern with no openings between the pin receptacles and the outer surface of the wax pattern; d. forming a ceramic mold surrounding the wax pattern; e. removing the wax pattern from the ceramic mold; f. casting the hollow investment casting in the ceramic mold, thus forming a casting having protrusions with excess material in the pin cavity portions; g. removing the metal article from the ceramic mold; h. removing the mini cores from the metal article; and i. removing the excess material from the protrusions, thus exposing the ends of the connecting holes. 